A. Field of the Invention
The present invention relates to a power semiconductor device used in a device such as a power conversion device and a manufacturing method thereof, and particularly to a process of forming an isolation layer in a bidirectional device or a reverse-blocking device having bidirectional voltage withstanding characteristics.
B. Description of the Related Art
In a semiconductor device of a reverse-blocking type, reverse-blocking capability equivalent to forward-blocking capability is required. In order to secure reverse-blocking capability, it is necessary to make a p-n junction holding a reverse breakdown voltage extend from the bottom surface of a semiconductor chip to its top surface. A diffused layer for forming the p-n junction extended from the top surface to the bottom surface is an isolation layer.
FIGS. 28A to 28C are cross sectional views showing conventional manufacturing steps in a related manufacturing method in the order in a case of forming an isolation layer in a related reverse-blocking IGBT presented by its principal part. The method is one of forming the isolation layer by coating and diffusion. First, on semiconductor wafer 151, oxide film 152, having a film thickness of about 2.5 μm is formed by thermal oxidation as a dopant mask (FIG. 28A). Next, oxide film 152 is subjected to patterning etching, by which opening 153 is formed for forming an isolation layer (FIG. 28B).
Following this, opening 153 is coated with boron source 154. Thereafter, a high temperature and lengthy heat treatment of semiconductor wafer 151 is carried out in a diffusion furnace to form a p-type diffused layer with a thickness on the order of several hundred micrometers (FIG. 28C). The p-type diffused layer becomes isolation layer 155. Then, although not particularly illustrated, after a top surface structure is formed, the bottom surface of semiconductor wafer 151 is ground until ground surface 156 reaches isolation layer 155 to thin semiconductor wafer 151. On ground surface 156, a bottom surface structure is formed which is made up of a p collector region and a collector electrode. Subsequent to this, semiconductor wafer 151 is cut at a scribing line positioned at the center of separation layer 155 to form an IGBT chip.
FIG. 29 is a cross sectional view showing a principal part of the related reverse-blocking IGBT whose isolation layer 155 is formed by the method shown in FIGS. 28A to 28C. In FIG. 29, reference numeral 161 denotes a p well region, 162 denotes a p voltage withstanding region, 163 denotes an emitter region, 164 denotes a gate insulator film, 165 denotes a gate electrode, 166 denotes an interlayer insulator film, 167 denotes an emitter electrode, 168 denotes a field oxide film, 169 denotes a field plate, 170 denotes a p collector region, 171 denotes a collector electrode, and 172 denotes a dicing face.
FIGS. 30A to 30C are cross sectional views showing manufacturing steps in order in another conventional case of forming an isolation layer in a related reverse-blocking IGBT presented by its principal part. In this method the isolation layer is formed by providing a trench and forming a diffusion layer on the side wall of the trench. First, an etching mask is formed with thick oxide film 173 having a thickness of several micrometers (FIG. 30A). Next, a trench having a depth of the order of several hundred micrometers is formed by carrying out dry etching (FIG. 30B). Then, an impurity is introduced into the side wall of the trench by means of vapor phase diffusion 175 to form isolation layer 176 (FIG. 30C).
FIG. 31 is a cross sectional view of a principal part of the related reverse-blocking IGBT in which isolation layer 176 is formed by the method shown in FIGS. 30A to 30C. Trench 174 is filled with reinforcing material 177. Thereafter, dicing is carried out along a scribing line, by which an IGBT chip is cut from semiconductor wafer 151. In this way, a reverse-blocking IGBT is completed. Reference numeral 178 denotes a dicing face. The other constituents are the same as those shown in FIG. 29.
Such a method of providing trench 174 and forming isolation layer 176 on the side wall of trench 174 is disclosed in JP-A-2-22869, JP-A-2001-185727 and JP-A-2002-76017. In JP-A-2-22869, it is disclosed that a trench is formed from the top surface of a device to a bottom side junction so as to surround an active layer, and a diffusion layer is then formed on the side face of the trench to form an isolation layer with an end of the bottom side junction of the device extended to the top surface of the device. In JP-A-2001-185727 and JP-A-2002-76017, it is disclosed that, as in JP-A-2-22869, a trench is formed from the top surface of the device to a bottom junction and a diffusion layer is then formed on the side face of the trench to thereby make the device provided as a device having a reverse-blocking capability.
In a method of forming the isolation layer in the reverse-blocking IGBT shown in FIGS. 28A to 28C, a high temperature and lengthy diffusion treatment is necessary for diffusing boron by carrying out heat treatment from boron source 154 (a liquid diffusion source of boron) coated on the surface to form isolation layer 155 with a diffusion depth of the order of several hundred micrometers. This makes quartz fixtures forming a diffusion furnace such as a quartz board, a quartz tube and a quartz nozzle necessary. Such fixtures cause fatigue, contamination by foreign materials from a heater and strength reduction due to devitrification of the quartz fixtures.
Moreover, in forming isolation layer 155 by the coating and diffusion method, it becomes necessary to form a masking oxide film (oxide film 152). The masking oxide film is required to be provided as a thick oxide film with a high quality for being made to withstand the lengthy boron diffusion. As a method of obtaining a silicon oxide film with high resistance of mask, that is, with a high quality, there is a thermal oxidation method.
However, it is necessary to form a thermal oxide film with a film thickness of about 2.5 μm in order to prevent boron atoms from penetrating through the masking oxide film during the diffusion processing of isolation layer 155 with boron, which takes place at high temperature for a long time, e.g., at 1300° C. for 200 hours. For forming such a thermal oxide film with a film thickness of about 2.5 μm, an oxidation time required at an oxidation temperature of 1150° C., for example, is about 200 hours in dry oxidation (dry atmosphere of oxygen), by which a high quality oxide film can be obtained.
Even with wet or pyrogenic oxidation, which is known to require a shorter oxidation time compared with that in dry oxidation though there is slight inferiority in quality of an obtained oxidized film, a long oxidation time of about 15 hours is still necessary. Furthermore, in the above oxidation processing, a large amount of oxygen is introduced into a silicon wafer. This introduces crystal defects such as oxygen deposits and oxidation induced stacking faults (OSF) and produces oxygen donors to thereby cause adverse effects such as characteristics deterioration and reliability degradation of a device.
Furthermore, also in the step of diffusing boron carried out after boron source 154 has been coated, the above high temperature and lengthy diffusion processing is usually carried out under an atmosphere of oxygen. This causes oxygen atoms to be introduced into crystal lattices in the wafer as interstitial oxygen atoms. Thus, also in the diffusion step, crystal defects such as oxygen deposits, oxygen donor production, OSF and slip dislocations are introduced. It is known that a leakage current is increased in a p-n junction formed in a wafer with such crystal defects being introduced and a breakdown voltage and reliability are significantly degraded in an insulator film formed on the wafer by thermal oxidation. Moreover, oxygen atoms taken in during diffusion processing become donors to cause an adverse effect of lowering a breakdown voltage.
In the method of forming the isolation layer shown in FIGS. 28A to 28C, approximately isotropic diffusion of boron progresses toward a silicon bulk from the opening of the masking oxide film. Thus, boron diffusion of up to 200 μm in the depth direction causes the boron to be inevitably diffused also in the lateral direction up to 160 μm. This causes an adverse effect on reduction in device pitch and chip size.
In the method of forming the isolation layer shown in FIGS. 30A to 30C, trench 174 is formed by dry etching and boron is introduced into the side wall of the formed trench 174 to form the isolation layer. Thereafter, trench 174 is filled with reinforcing material 177 such as an insulator film or semiconductor film. Since a trench with a high aspect ratio can be formed, the formation method shown in FIGS. 30A to 30C is more advantageous for reduction in device pitch as compared to the forming method shown in FIGS. 28A to 28C.
However, the processing time required for etching to a depth of the order of 200 μm is on the order of as long as 100 minutes per one wafer when a typical etching equipment is used. This brings adverse effects such as an increase in lead time and the amount of maintenance. Moreover, when a deep trench is formed by dry etching with a silicon oxide (SiO2) film used as a mask, a thick silicon oxide film with a thickness of several micrometers is necessary because the etching selectivity is 50 or less. The thick silicon oxide film causes adverse effects such as increase in cost, reduction in a rate of acceptable products due to introduction of process-induced crystal defects such as OSFs and oxygen deposits.
Further, when a process of forming an isolation layer in which a deep trench with a high aspect ratio that is formed by dry etching is used, there is a problem in that residues such as chemical residue 179 and resist residue 180 are left in the trench as shown in FIG. 32 to cause adverse effects such as reduction in yield and reduction in reliability. When a dopant such as phosphorus or boron is introduced into the side wall of a trench, dopant introduction usually is carried out by implanting dopant ions with the wafer inclined because of the vertical side wall of the trench. However, introduction of a dopant into the side wall of the trench having a high aspect ratio causes adverse effects such as reduction in an effective dose (and an accompanying increase in implantation time), a decrease in effective projected range, a dose loss due to presence of a screen oxide film and reduction in implantation uniformity. Therefore, in order effectively to introduce an impurity into a trench having a high aspect ratio, a vapor phase diffusion is used in which a wafer is exposed to an gasified atmosphere of a dopant such as PH3 (phosphine) or B2H6 (diborane) instead of implanting dopant ions into an wafer. The vapor phase diffusion, however, is inferior in fine controllability of dose compared with ion implantation.
Moreover, when a trench having a high aspect ratio is filled with an insulator film, a space referred to as a void is produced in the trench which causes a reduction in reliability. A method previously has been proposed in which a trench is formed with anisotropic dry etching and then boron is diffused from the inner face of the trench to form an isolation layer (Japanese Patent Application No. 2004-36274). By the proposed method, the spread of boron in the lateral direction in a wafer can be inhibited. Furthermore, in the methods disclosed in each of the above-described JP-A-2-22869, JP-A-2001-185727 and JP-A-2002-76017, it is conceivable that a step of filling a trench with a reinforcing material may be necessary for cutting a wafer at a scribing line to provide a semiconductor chip and a manufacturing cost is therefore increased.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.